The present invention relates generally to a method and apparatus for use in a communication network and, in particular, to a method and apparatus for reducing audio gaps during a handoff in a communication network.
Digital or analog communication networks such as cellular or personal communication services (PCS) networks include infrastructure hardware that produces cells of coverage in which communication services are provided. A number of cells may overlap or abut one another to provide coverage over a significant geographical area. A user located within a cell may have access to the communications network via a mobile unit, such as a hand held portable telephone or a car telephone or the like. The mobile unit may communicate with the communication network using predetermined frequencies or digital codes associated with a particular cell with which the mobile unit is communicating. The communication network may be coupled to a conventional public switched telephone network (PSTN) to enable land line users (i.e., conventional terrestrial telephone users) to exchange information with mobile unit users.
As a user operating a mobile unit moves from one cell (e.g., a source cell) to another cell (e.g., a target cell), communication handoffs occur within both the infrastructure hardware and the mobile unit. During such a handoff, the mobile unit disconnects from the infrastructure hardware at the source cell and connects to the infrastructure hardware at the target cell. In the process of disconnecting and connecting the mobile unit may perform frequency tuning or may change digital codes to enable communication with the infrastructure at the target cell. While the mobile unit disconnects from the source cell and connects to the target cell, the infrastructure hardware at the source cell prepares to end communications with the mobile unit and the infrastructure hardware at the target cell prepares to begin communicating with the mobile unit. The timing at which the handoffs in the mobile unit and the infrastructure occur may lead to audio gaps in inbound audio (i.e., audio from the mobile unit to the infrastructure) and outbound audio (i.e., audio from the infrastructure to the mobile unit).
A portion of a prior art communication network 10 in the process of a handoff is shown in FIG. 1. The communication network 10, which may be a cellular network, includes a number of base transceiver stations (BTS""s) (only two of which are shown) 14, 16, each of which provides a cell of coverage 18, 20, respectively. Each BTS 14, 16 is selectively interfaced to a base site controller (BSC) 22, which is further interfaced to a mobile switching center (MSC) 23. The MSC 23 communicatively couples the BSC 22 to a PSTN 24.
Each base site controller (e.g., the BSC 22) may provide communication service to one or more BTS""s 14, 16. The BSC 22 may include one or more transcoders or voice processors 26, 28 that process communication information that is exchanged between the MSC 23 and a mobile unit 36, which may be disposed within one of the cells (e.g., the cell 18). Each of the voice processors 26, 28 may have an associated switch 30, 32, which may be controlled by a central processing unit (CPU) 34. The voice processors 26, 28 are selectively interfaced to the MSC 23 by the switches 30, 32. The BSC 22 may provide message transfer and call switching functionality and may be controlled by the MSC 23, via the CPU 34.
As shown in FIG. 1, the mobile unit 36 is near the interface of the cell 18 and the cell 20. While the mobile unit 36 is within the cell 18, communications are handled exclusively by the voice processor 26 as represented by the solid lines connecting the voice processor 30 and the BTS 14, via the switch 30. At such time as the mobile unit 36 traverses from the cell 18 to the cell 20 (a determination that is typically made by both the MSC 23 and the mobile unit 36), a handoff takes place. During a handoff, the switch 30 associated with the voice processor 26 is controlled by the CPU 34 to disconnect the voice processor 26 from the BTS 14 and to connect the voice processor 26 to the BTS 16 in both the inbound and outbound directions, such a connection is represented by the dashed lines in FIG. 1. In addition to the switching in the BSC 22, the mobile unit 36 switches from a frequency or code corresponding to the cell 18 to a frequency or code corresponding to the cell 20.
Ideally, the switch 30 disconnects from the BTS 14 and connects to the BTS 16 at the exact time the mobile unit 36 switches from the cell 18 to the cell 20 because the voice processor 26 can only process one audio source (e.g., one BTS 14 or 16) at a time. However, in reality this switching is not synchronous. Accordingly, for the communication network 10 shown in FIG. 1, audio will be interrupted by an audio mute. The mute occurs in both the outbound path (i.e., the path from the MSC 23 to the mobile unit 36) and the inbound path (i.e., the path from the mobile unit 36 to the MSC 23). The duration of the mute is the length of time between when the switch 30 switches and when the mobile unit 36 switches.
A known communication network 60 that eliminates an outbound audio mute during a handoff is shown in FIG. 2. Like the communication network 10 shown in FIG. 1, the communication network 60 includes a number of BTS""s (only two of which are shown) 64, 66, each of which provides a cell of coverage 68, 70, respectively. Each BTS 64, 66 is selectively interfaced to a BSC 74, and each BSC 74 typically provides communication service to one or more BTS""s 64, 66. The BSC 74 typically includes one or more voice processors 76, 78, switches 80, 82 and a central processing unit (CPU) 84. The voice processors 76, 78 process communication information that is sent to and received from a mobile unit 86. The voice processors 76, 78 are selectively interfaced to an MSC 90, via the switches 80, 82. The MSC 90 provides an interface between the BSC 74 and a PSTN 94. The CPU 84 is provided to control the switches 80, 82.
While the mobile unit 86 is within the cell 68, communications are handled by the voice processor 76, as represented by the solid lines connecting the voice processor 76, the switch 80 and the BTS 64. The outbound connection between the voice processor 76 and the BTS 64 is coupled through the switch 80, which is adapted to selectively connect to either or both of the BTS""s 64, 66. As the mobile unit 86 traverses near the cell 70 (as shown in FIG. 2) the MSC 90 determines that a handoff is likely to occur and the CPU 84 controls the switch 80 to establish a link from the voice processor 76 to the BTS 66. Such a situation is shown by the dashed line from the switch 80 to the BTS 66 in FIG. 2. Such a link provides outbound audio to the cell 70 before the mobile unit 86 reaches that cell. Thus, when the mobile unit 86 reaches the cell 70, outbound audio will already be present at the cell 70. Such a configuration eliminates any outbound audio mute.
The configuration shown in FIG. 2 does not, however, eliminate an inbound audio mute caused by the fact that the switch 80 does not switch its inbound connection from the BTS 64 to the BTS 66 until the handoff actually occurs. When the mobile unit 86 is in the cell 68, the connection from the BTS 64 to the switch 80 is in use, as represented by the solid line from the BTS 64 to the switch 80.
When the mobile unit 86 moves into the cell 70, communication is established from the BTS 66 to the switch 80 and the voice processor 76, as represented by the dashed line from the BTS 66 to the switch 80 and the voice processor 76. Additionally, the connection from the BTS 64 to the voice processor 76 may be broken or xe2x80x9ctorn down.xe2x80x9d The voice processor 76 is only connected to either the BTS 64 or the BTS 66 because the voice processor 76 can only process one inbound signal at a time (e.g., either the signal from the BTS 64 or the signal from the BTS 66). The mobile unit 86 does not switch from the frequency or code associated with the cell 68 to the frequency or code associated with the cell 70 until a handoff takes place. As with the communication network 10 of FIG. 1, ideally when a handoff takes place, the switch 80 disconnects its inbound path from the BTS 64 and connects its inbound path to the BTS 66 at exactly the same time the mobile unit 86 switches. However, in reality this is rarely possible and, therefore, an inbound audio mute having a duration equal to the difference in switching times between the switch 80 and the mobile unit 86 results.
Therefore, there is a need for a method and an apparatus for reducing audio gaps during a handoff in a communication network.